---
title: 'Fastify'
description: How to use ts-rest with Fastify
---

## Installation

<InstallTabs packageName="@ts-rest/fastify" />

## Usage

```typescript
import Fastify from 'fastify';
import { initServer } from '@ts-rest/fastify';

const app = Fastify();

const s = initServer();

const router = s.router(contract, {
  getPost: async ({ params: { id } }) => {
    const post = await prisma.post.findUnique({ where: { id } });

    return {
      status: 200,
      body: post,
    };
  },
  createPost: async ({ body }) => {
    const post = await prisma.post.create({
      data: body,
    });

    return {
      status: 201,
      body: post,
    };
  },
});

app.register(s.plugin(router));

const start = async () => {
  try {
    await app.listen({ port: 3000 });
  } catch (err) {
    app.log.error(err);
    process.exit(1);
  }
};

start();
```

`s.registerRouter` is a function that takes a contract, a corresponding router with implementations for each route and a fastify app, and it will
create the corresponding fastify routes for each endpoint with the correct method, paths and middleware and attach them to your fastify app.

## Options

You can pass an optional options object as the last argument for `s.registerRouter`.

```typescript
type Options = {
  logInitialization?: boolean; // print route initialization logs to console
  jsonQuery?: boolean;
  responseValidation?: boolean;
  requestValidationErrorHandler?:
    | 'combined'
    | ((
        err: TsRestRequestValidationError,
        request: fastify.FastifyRequest,
        reply: fastify.FastifyReply,
      ) => void);
};
```

### Response Validation

To enable response parsing and validation, you can use the `validateResponses` option.
If there is a corresponding response Zod schema defined in the contract for the returned status code, the response will be parsed and validated.
If validation fails a `ResponseValidationError` will be thrown causing a 500 response to be returned.

```typescript
s.registerRouter(contract, router, app, {
  validateResponses: true,
});
```

### Request Validation Error Handling

The default functionality of handling request validation errors is `combined` which returns a 400 response with all validation errors in the body in this form.

```typescript
{
  pathParameterErrors: z.ZodError | null;
  headerErrors: z.ZodError | null;
  queryParameterErrors: z.ZodError | null;
  bodyErrors: z.ZodError | null;
}
```

You can also pass a custom error handler function to the `requestValidationErrorHandler` option.

```typescript
s.registerRouter(contract, router, app, {
  requestValidationErrorHandler: (err, req, res, next) => {
    //             err is typed as ^ RequestValidationError
    res.status(400).json({
      message: 'Validation failed',
    });
  },
});
```
